This invention relates to pressure senders which are designed to be tapped into a fluid line, to monitor the pressure in that line, and specifically to pressure senders that deliver an analog signal to a pressure gauge so that the pressure of the fluid line can be continuously monitored.
There are many applications in which system fluid pressure needs to be monitored. Liquid pressure is monitored, for example, for oil pressure in internal combustion engines, as well as in other automobile systems such as brake fluid systems. Air pressure needs to be monitored in air actuated brake systems and in any number of power tools which are driven by compressed air.
For safety reasons as well as to aid preventative maintenance, it is desirable in situations such as these to continuously monitor pressure rather than simply indicate when a low pressure condition has occurred. It is applications such as these in which pressure senders, rather than pressure switches, are used.
Typically, pressure senders are tapped into a pressure line so that a chamber of the pressure sender experiences the same pressure as that experienced by the pressure line. Pressure senders, also called transducers, receive a signal in one form and translate that signal to a corresponding signal in another form. With diaphragm actuated pressure senders, pressure changes in the pressure line cause a diaphragm in the sender to deflect, resulting in a mechanical signal. To turn this mechanical signal into an electrical signal, the diaphragm in turn displaces a follower which displaces an electrical contact of the pressure sender which is in contact with a variable resistor. As a result, the resistance of the pressure sender in the electrical circuit changes and that change is monitored by the pressure gauge which translates the change into a specific pressure value.
These known pressure senders are typically provided with wound copper wire rheostats. The rheostats are electrically wired to a terminal post at one end and to electrically insulative material at their other end. Typically the terminal post is where the current from the system flows to the pressure sender. When current passes from the terminal post to the rheostat, it will flow to the movable electrical contact. Such a pressure sender is described in U.S. Pat. No. 4,079,351 to Levine.
A problem with this type of pressure sender is that it is difficult to control the resistance of the rheostat at precise points along the coil. The result is that these known pressure senders are difficult to calibrate so that a desired resistance corresponds to a desired pressure. Also, it is difficult to manufacture the rheostats in mass with any uniformity so that all of the rheostats made have equivalent resistances at equivalent points. Due to this problem, it is difficult to construct pressure senders that use these rheostats with any uniformity. Each pressure sender has individual properties so that a pressure monitoring system using known pressure senders of this type has to be calibrated based on the properties of its individual pressure sender.
Another problem with known pressure senders using copper wire wound rheostats is that of mechanical breakdown through cycling. Due to a large friction surface of the moving electrical contact as it travels along the wire of the rheostat, both the electrical contact and the rheostat can deteriorate after extensive cycling. This deterioration can result in the pressure sender requiring re-calibration or in the complete failure of the device all together.
Known pressure senders have also experienced improper readings due to a poor electrical connection between the rheostat and the terminal post where current from the system flows to the pressure sender. The single wire connection which typically connects the rheostat to the terminal post can easily become dislodged due to a defect in manufacturing or to the pressure sender being subjected to mechanical shock.
As pressure senders of this type are often used in harsh environments leading to mechanical shock, it is important that the electrical circuit be securely established. In the past, pressure senders using copper wire wound rheostats have experienced problems in such environments due to the difficulty of adequately securing the rheostat connection to the rest of the electrical circuit. Also, as these copper wire wound rheostats are relatively heavy in comparison to the rest of the pressure sender, the force generated by the acceleration of the pressure sender during mechanical shock and acting on the mass of the rheostat can be great. This force often results in damage to the electrical circuit of the pressure sender.
It is, therefore, an object of the present invention to provide a pressure sender which can be manufactured more efficiently and uniformly than known pressure senders.
It is also an object of the present invention to offer a pressure sender which is more reliable in harsh environments than known pressure senders.
It is another object of the present invention to offer a pressure sender that is more durable and will have a longer service life than known pressure senders.
It is yet another object of the present invention to provide a pressure sender which offers more consistent and accurate signals than known pressure senders.